Transcript

5.
The current flows from the tungsten electrode to the work surface, and thepositively charged argon gas ions flow from the work surface to the tungstenwhich puts too much heat into the metal and causes the base metalunderneath the oxide layer to liquefy while the surface remains hard andimpenetrable.The electrode is connected to the negative terminal of the power supply.Electrons are emitted from the tungsten electrode and accelerated whiletraveling through the arc. A significant amount of energy, called the workfunction, is required for an electron to be emitted from the electrode. When theelectron enters the workpiece, an amount of energy equivalent to the workfunction is released. This is why in GTAW with DCEN more power (about two-thirds) is located at the work end of the arc and less (about one-third) at theelectrode end. Consequently, a relatively narrow and deep weld is produced. 20

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Direct current electrode posititive (DCEP) solves the oxide problem becausethe current flows from the workpiece to the tungsten, lifting the oxide off thematerial in the arc zone. DCEP alone provides the oxide cleaning action andvery little penetration. Because the heat is concentrated on the tungsteninstead of the workpiece, DCEP also causes the tungsten to ball up at the end.Direct-Current Electrode Positive (DCEP) This is also called the reversepolarity. The electrode is connected to the positive terminal of the powersource. The heating effect of electrons is now at the tungsten electrode ratherthan at the workpiece. Consequently, a shallow weld is produced.Furthermore, a large-diameter, water-cooled electrodes must be used in orderto prevent the electrode tip from melting. The positive ions of the shielding gasbombard the workpiece, knocking off oxide ﬁlm and producing a clean weldsurface. Therefore, DCEP can be used for welding thin sheets of strong oxide-forming materials such as aluminum and magnesium, where deep penetrationis not required. 21

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AC, then, combines DCEN and DCEP to provide good heat penetration withcleaning action.The oxide is sandblasted away – as you can see in the following pictures. 22

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Historically, though, AC has posed an obstacle to GTAW because the arcfrequently extinguishes itself as the current reaches a zero point beforereversing directions. Without any current passing between the tungsten andthe base metal, the arc simply goes out and affects the quality of the weldingarc – and the quality of the weld. 26

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The introduction of Squarewave technology brings a new level of TIG-weldingperformance to the market.Improvements in transformer-based GTAW machines created the square wave, which increased the amount of time the arc spends at full-current flow in both DCEN and DCEP. Square-wave technology eliminated the tendency for the arc to extinguish when the current came to a halt as it reversed directions by making the transition very quickly. This greatly improved the stability of the arc and made square-wave technology the preferred method for GTAW of aluminum and other materials that form an oxide layer, such as magnesium. 27

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This wave is balanced with both cleaning and penetration.Independent amperage (or amplitude) control allows the EP and EN amperages to be set independently. This precisely controls heat input into the work and even takes heat off the electrode. The EN portion of the cycle controls the level of penetration, and the EP portion affects the arc cleaning action. 28

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Here, you can see the difference in welds between maximum cleaning andmaximum penetration.A current with greater EN than EP creates a narrow bead with deeperpenetration and no visible cleaning action, ideal for fillet welds and automatedapplications. A current with greater EP than EN gives the operator a widerbead with less penetration and clearly visible cleaning action, ideal for buildupwork 29

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This controls the amount of arc-cleaning action and width of the arc-etchingzone around the weld. 30

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The second major revolution in frequency technology came with the invention of theinverter, which created the ability to increase or decrease output frequency beyondthe standard 60 Hz, which is the standard frequency delivered to every outlet in theU.S. (other countries, such as Germany, England, and France, deliver AC power at50 Hz). The inverter also allowed for the development of the advanced square wave,which decreases the time it takes for the current to reverse directions, increasing arcstability even more and eliminating the need for continuous high frequency.Increasing frequency above 60 Hz causes the current to change direction more often, which means that it spends less time per cycle in both DCEN and DCEP mode. By spending less time at each polarity, the arc cone has less time to expand.An arc cone at 400 Hz is much tighter and more focused at the exact spot the electrode is pointing than an arc cone operating at 60 Hz (see Figure 1). The result is significantly improved arc stability, ideal for fillet welds and other fit-ups requiring precise penetration.Combined with adjustable balance control to increase the electrode negative polarity—resulting in deeper penetration and tungsten that doesnt ball up—high AC frequency can weld very tight joints with good penetration and without the risk of laying down too much filler metal. Workpieces with wide gaps to fill or that require buildup will benefit from the softer, wider arc cone that results from lower frequenciesUnlike other types of waveform control, such as balance and amplitude, frequency control provides good penetration at both low and high frequencies. The primary difference between the two is the width of the arc cone and resulting weld bead. 31

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Controls the width of the arc cone. Increasing the AC Frequency provides amore focused arc with increased directional control.Decreasing the AC Frequency softens the arc and broadens the weld puddlefor a wider weld bead.A Good starting point for working with adjustable frequency for the first time isbetween 80 and 120 Hz. 32

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At 60 Hz, you can see the bead doesn’t quite penetrate the thick aluminum. 33

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At 200 Hz, the bead is much tighter and penetrated the thicker metal. 34

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Arc shaping capabilities are enhanced by improved balance control. On theleft is the tungsten with a balled end, due to more time spent in the electrodepositive (EP) part of the cycle, which creates shallower penetration. On theright is the tungsten with a sharp end, due to more time spent in the electrodenegative (EN) part of the cycle, which creates deeper penetration and allowsfaster travel speeds. 37

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The four different waveforms affect the arc and puddle characteristics and thepenetration profile.An advanced square wave (A) allows for fast travel speed. The advanced square wavewaveform offers fast transitions between EN and EP for aresponsive, dynamic, and focused arc with better directional control. It forms a fast-freezing puddle with deep penetration and fast travel speeds.A soft square wave (B) provides maximum puddle control. Soft square waveprovides a smooth, soft, "buttery" arc with a fluid puddle and good wettingaction. The puddle is more fluid than with advanced square wave and morecontrollable than with sine wave.The sine wave (C) permits welding with traditional characteristics. The sinewave offers a soft arc with the feel of a conventional power source. It providesgood wetting action and actually sounds quieter than other waves. Its fasttransition through the zero amperage point also eliminates the need forcontinuous high frequency.The triangular wave (D) reduces heat input. The triangular wave offers peakamperage while reducing overall heat input into the weld. This leads to quickpuddle formation, low weld distortion, and fast travel speeds. It is especiallygood for welding thin aluminum. 39

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Independent AC Amperage Control allows the EN and EP amperage values to be set independently. Adjust the ratio of EN to EP amperage to precisely control heat input to the work and the electrode. EN amperage controls the level of penetration, while EP amperage dramatically effects the arc cleaning action along with the AC Balance control.For example, when welding a thick piece of aluminum, the operator can pour350 amps of EN into the weld and only 175 amps of EP into the tungsten.This allows faster travel speeds, faster filler metal deposition, deeperpenetration, and the potential to eliminate preheating. Case studies aboutGTAW inverters with independent amperage control suggest that companiescan cut production time by as much as two-thirds.Increasing EN while maintaining or reducing EP also permits the use of a smaller-diameter tungsten. This takes heat off of the tungsten and more precisely directs it into the weld. Companies have reported that this has allowed them to purchase thinner-diameter electrodes, which are less expensive than the thicker variety. 42